New suprathermal proton population around the Moon: Observation by SARA on Chandrayaan-1

Abstract

We report a new population of suprathermal H+ (∼1.5–3 times the solar wind energy) around the Moon observed by Solar Wind Monitor (SWIM)/Sub-keV Atom Reflecting Analyzer (SARA) on Chandrayaan-1. These ions have large initial velocity (>100 km s−1) and are observed on the dayside, near the terminator, and in the near-wake region (100–200 km above the surface), when the Moon is located outside Earth's bow shock. Backtracing suggests that the source is located >500 km above the dayside lunar surface. Possible sources considered for these ions are ionization of backscattered lunar energetic neutral hydrogen atoms, ionization of lunar exospheric hydrogen, inner source pickup ions, interstellar pickup ions, ionization of neutral solar wind, and solar wind H+ reflected from Earth's bow shock. The comparison of the observed flux (density) with that expected from these sources and the velocity distribution suggests that an additional source is required to explain the population.